It is known, as described in Japanese Patent Application (OPI) No. 131654/81, for example, that an acryl resin is useful as a material for production of transparent optical articles because of its excellent properties such as (1) good transparency, (2) good fluidity and (3) low double refraction (the term "OPI" as used herein means a published unexamined Japanese patent application). Such acryl resins, however, have disadvantages in that heat resistance is as low as about 70.degree. C., impact resistance is low, and may wrap on absorbing water.
In order to eliminate these disadvantages of acryl resins, a polycarbonate resin having a viscosity average molecular weight of from 15,000 to 18,000 is now under investigation as a material for production of optical discs and lenses, as disclosed in Japanese Patent Application (OPI) No. 180553/83. Such polycarbonate resins, however, still have disadvantages in that the fluidity is insufficient and the double refraction as a most important factor in producing optical articles is still high. This poor fluidity has been improved by decreasing the molecular weight of the polycarbonate resin, or by increasing a molding temperature.
Although the improvement in the fluidity is practically useful, it causes different problems such as insufficient mechanical strength and stringiness during the molding process thereby making it impossible to mold articles continuously. Thus, the polycarbonate resin is still limited in its practical use. In the precision optical system used in reading or writing of information utilizing laser, it has been desired to develop optical materials having a low double refraction, a high mechanical strength, and being free from stringiness capable of being molded continuously. The term "stringiness" as used herein means a phenomenon that a molten resin remains between a nozzle and an end of a sprue, and it sticks to the sprue in a string form in taking out a molded article.
The double refraction of a transparent optical article as described above varies with properties of the resin material used and also with molding conditions.
In molding a transparent optical article, a resin is melted and cooled in a mold. If the viscosity of the molten resin is too high, the resin is cooled in a non-uniform state whereby optical strain remains in the resulting molded article which then gives rise to undesired double refraction. Particularly, if the viscosity of the resin used in the injection molding is too high, orientation of the resin remains in a flow direction since the resin is injected in a mold whereby double refraction is liable to develop in the molded article.
As a means for moderating molding conditions, a conventionally known technique, that is, a method for compounding plasticizers to prepare a molding material of high fluidity can be considered. However, when conventional plasticizers for polycarbonate resins such as olefin- and phosphate-based plasticizers are added in an amount sufficient to improve moldability, fluidity can be improved, but the desired optical molded article cannot be obtained due to contamination of mold with the plasticizer which results in the production of contaminated molded articles, or poor compatibility between the plasticizer and the polycarbonate resin lowers the transparency which results in the production of molded articles having poor appearance. Moreover, physical properties are reduced to an extent that is not practically acceptable.
For this reason, the reduction in double refraction of transparent optical articles has been considered by changing molding conditions, particularly increasing a molding temperature, or by using a low molecular weight polycarbonate resin having good fluidity. However, when the low molecular weight polycarbonate resin is molded at high temperatures, the resulting molded article is reduced in double refraction but is poor in strength and causes stringiness in injection molding. Thus, a material which is satisfactory in total aspects of double refraction, strength and continuous moldability has not yet been discovered.